+When a UDP socket is created, its local and remote addresses are unspecified. Datagrams can be sent immediately using sendto(2) or sendmsg(2) with a valid

+destination address as an argument. When connect(2) is called on the socket the default destination address is set and datagrams can now be sent using send(2) or

+write(2) without specifying an destination address. It is still possible to send to other destinations by passing an address to sendto(2) or sendmsg(2).

+In order to receive packets the socket can be bound to an local address first by using bind(2). Otherwise the socket layer will automatically assign a free local port out of the range defined by ''net.ipv4.ip_local_port_range'' and bind the socket to ''INADDR_ANY''.

-This
is an implemention of
the User Datagram Protocol

-described in RFC768. It implements a connectionless
,

-unreliable datagram
packet service. Packets may be reordered

-or duplicated before they arrive. UDP generates
and checks

-checksums to catch transmission errors
.

+All receive operations return only one packet. When the packet
is smaller than
the passed buffer only that much data is returned
, when it is bigger the
packet is truncated
and the __MSG_TRUNC__ flag is set. ''MSG_WAITALL'' is not supported
.

+IP options may be sent or received using the socket options described in ip(7). They are only processed by the kernel when the appropriate sysctl is enabled (but still

+passed to the user even when it is turned off). See ip(7).

-When a UDP socket
is created, its local and remote addresses

-are unspecified. Datagrams can be sent immediately using

-sendto(2) or sendmsg(2) with a valid

-destination address as an argument. When connect(2)

-is called
on the socket the default
destination address is

-set and datagrams can now be sent using send(2) or

-write(2) without specifying an destination address.

-It is still possible to send
to other destinations by

-passing
an address to sendto(2) or sendmsg(2).

-In order to receive packets the socket can be bound to an

-
local address first by using bind(2). Otherwise the

-socket layer will automatically assign a free local port out

-of the range defined by ''net.ipv4.ip_local_port_range''

-
and bind
the socket
to ''INADDR_ANY''
.

+When the __MSG_DONTROUTE__ flag
is set
on sending
the destination address must refer
to an local interface
address and the packet is only sent
to that interface
.

+UDP fragments a packet when its total length exceeds the interface [MTU] (Maximum Transmission Unit). A more network friendly alternative is to use path MTU discovery as described in the __IP_PMTU_DISCOVER__ section of ip(7).

-All receive operations return only one packet. When the

-packet is smaller than the passed buffer only that much data

-is returned, when it is bigger the packet is truncated and

-the __MSG_TRUNC__ flag is set. ''MSG_WAITALL'' is not

-supported.

-

-

-IP options may be sent or received using the socket options

-described in ip(7). They are only processed by the

-kernel when the appropriate sysctl is enabled (but still

-passed to the user even when it is turned off). See

-ip(7).

-

-

-When the __MSG_DONTROUTE__ flag is set on sending the

-destination address must refer to an local interface address

-and the packet is only sent to that interface.

-

-

-UDP fragments a packet when its total length exceeds the

-interface MTU (Maximum Transmission Unit). A more network

-friendly alternative is to use path MTU discovery as

-described in the __IP_PMTU_DISCOVER__ section of

-ip(7).

!!ADDRESS FORMAT

+UDP uses the IPv4 __sockaddr_in__ address format described in ip(7).

-

-UDP uses the IPv4 __sockaddr_in__ address format

-described in ip(7).

!!ERROR HANDLING

+All fatal errors will be passed to the user as an error return even when the socket is not connected. This includes asynchronous errors received from the network. You may get an error for an earlier packet that was sent on the same socket. This behaviour differs from many other BSD socket implementations which don't pass any errors unless the socket is connected. Linux's behaviour is mandated by RFC:1122 .

+For compatibility with legacy code it is possible to set the __SO_BSDCOMPAT__ SOL_SOCKET option to receive remote errors only when the socket has been connected (except for __EPROTO__ and __EMSGSIZE__). It is better to fix the code to handle errors properly than to enable this option. Locally generated errors are always passed.

-All fatal errors will be passed to
the user as an error

-return even when the socket
is not connected. This includes

-asynchronous
errors received from
the network. You may get

-an
error for an earlier packet that was sent on
the same

-socket. This behaviour differs from many other BSD socket

-implementations which don't pass any errors unless the

-socket is connected. Linux's behaviour is mandated by

-
__RFC1122
__.

+When
the __IP_RECVERR__ option
is enabled all
errors are stored in
the socket
error queue and can be received by recvmsg(2) with
the __MSG
_ERRQUEUE
__ flag

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